Electrical contact to semiconductor body



Sept. 19, 1961 s. PESSOK ETAL 3,000,798

ELECTRICAL CONTACT TO SEMICONDUCTOR BODY Filed June 9, 1959 55 M '972WAE4/ P551905, 70 3 7 MAM/a8. MEL/LLQ, W Y INVENTORS, 2 172;: 10

7 I ml BY Maw/M 8f 92 United States Patent 3,000,798 ELECTRICAL CONTAfiCT T SEMICONDUCTOR ODY Stanley Pessok, Rolling Hills, and Manlio B. Melillo, Inglewood, Califi, assignors to Pacific Semiconductors, Inc., Culver 'City, Calif., a corporation of Delaware Filed June 9, 1959, Ser. No. 819,142 2 Claims. (Cl. 204-15) This invention relates to semiconductive devices and more particularly to a method for fabricating large area low resistance contacts to the surface of a body of semiconductive material.

It has long been desirable for many types of semiconductive devices to make contact thereto in a manner which is advantageous both electrically and mechanically in order to produce devices which are capable of .carrying relatively large currents. In order to produce a device having a large current capacity it has been found that the efiiciency of such a device hinges materially upon the electrical resistance of the contact. It has therefore, been found necessary to produce an electrical contact combining good mechanical strength with a resistivity of the order of one-thousandth of an ohm. It has also been desired to produce a contact of the character described which is resistant to etchants :typically used in the semiconductor industry, such as one consisting of a combination of acids including nitric acid, acetic acid and hydrofluoric acid.

Materials which have been found to be inherently capable of producing a low resistance ohmic contact include base metals such as tin, copper, and cadmium, and noble metals such as gold, silver, platinum 'and rhodium. The method-of the present invention has been found to be particularly successful in producing an electrical contact using gold, although it is not necessarily intended to be limited thereto. Prior art methods for depositing a layer of gold onto the surface of a semiconductive body such as silicon, for example, have included evaporative vplating, chemi-plating and electro-plating using local contacts.

=Evaporative plating involves certain disadvantages for the purposes herein stated, in that it requires expensive equipment and presents difficulty in producing uniform layers, especially layers having a controlled average thickness. Chemi-plating in the past, has for the purposes herein mentioned, proved to be .unsatisfactoryin that the layers produced have typically been too thin. Finally, electro plating using localcontacts producesuneven plating due to the fact that these local contacts, when coupled'with the relatively poor conductivity of semiconductor material,- results .in a non-uniform current density across .the surface to be plated.

.It is therefore an object of .the present invention to provide .an improved method for producing a layer of uniform thicknessof aconductin-g metal over a predetermined portion of a surface of a body of semiconductive material.

Another object of the presentinvention is to produce a low resistance broad areaetch resistant contact over a predeterminedportion ofthe surface of a semiconductivebody.

A further objectof the present invention is to provide a method 'for producing a uniform large area gold contact over apredetermined portion of the surface of a semiconductive'body.

Afurther object of the present invention is to produce a uniform'large area gold contact to the surface of a silicon "body.

A still further object of the present invention is to provide a method for producing a large arealow resistance ohmic contact of the character described which may be deposited in accordance with a predetermined pattern on the surface of a semiconductive body.

Yet another object of the present invention is to provide a large area ohmic contact of the character described which permits precise control of the average thickness of the deposited conducting metal.

A still further object of-the present invention 'is to provide a method for producing a low resistance ohmic contact of the character described which permits build-up of uniform relatively heavy layers of the deposited contacting metal.

Yet another object of the present invention is to provide a method for producing a contact of the character described which requires only normal electro-plating equipment and therefore renders it relatively inexpensive to practice.

This invention will be described with silicon as the semiconductive material to which contact is to be made and while the method hereinafter to be described is particularly suited to the formation of contacts to silicon of the character described, it is also applicable to germanium or silicon-germanium alloy as the semiconductive material.

This invention is .based, to a considerable extent, upon the discovery that improved large area contacts to silicon can be achieved by electro-plating a metal such as gold in two distinct steps. As a preliminary step a coating of silver is first deposited over the surface of the silicon body by wetting such body with a solution of argentous fluoride. Thereafter gold is electro-plated over the desired portion of the silicon body 'where contact is to .be made.

Thus, in accordance with the presently preferred embodiment of this invention, a large area .gold ohmic contact is produced upon a silicon wafer by first heating the wafer to a temperature in the range from 50 C. to 60 C. With the wafer maintained .at this temperature a solution of argentous fluoride is brushed over the top surface of the wafer opposite that surface which rests upon a hot plate. (The argentous lfluoride solution is prepared by dissolving argentous fluoride in distilled water so that the ratio ofzthe argentous fluoride to distilled water is one part in about 121 parts by Weight.) The solution is permitted to dry upon the silicon wafer thus causing elemental silver of a thickness approximately 10" inches to plate out upon the silicon water by the mechanism known as ionic-displacement. A gold foil to which a wire'has been attached is then glued or otherwise affixed to a glassplate. The silicon wafer with the silver side down is now placed over the gold foil to effect a pressure contact therebetween. A vinyl plastic which serves as both a mask and a cement is applied about the periphery of the wafer to form a fillet between it and the glass plate. A negative bias is next applied to the silver coating through the gold foil and its attached .wire. With the bias voltage still applied and with the silicon wafer mounted upon the glass, the glass plate is placed within a warm potassium gold cyanide bath. A positively biased electrode is-disposed in the bath at a distance from the glass plate. Thus, gold from'the bath is electro-plated upon the surface of the silicon wafer -opposite the silver coating. The gold plate so deposited has been found to be extremely uniform with its thickness being determined by the temperature of the bath, the time ofimmersion and the amountof current flowing. After the electro-plating is completed the gold contact is preferably sintered by placing it into an oven for approximately 30 minutes with theoven maintained at .a temperature of approximately 450 C.

The novel features which are believed to be characteristic of the present invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawing. It is to be expressly understood, however, that the drawing is for the purpose of illustration and example only, and. it is not intended as a definition of the limits of the invention.

In the drawing:

FIGURE I is a perspective view ofa silicon diode in an intermediate stage of production prior to the fabrication of the contact thereto in accordance with the present invention;

FIGURE 2 is a cross-sectional view of the diode of FIGURE 1, somewhat enlarged in size;

FIGURE 3 is a cross-sectional view of the diode of FIGURE 2 upon which a coating of silver has been deposited in accordance with an intermediate step in carrying out the present invention;

FIGURE 4 is a perspective view showing the diode of FIGURE 3 mounted upon a glass plate which is deposited within an electro-plating tank;

FIGURE 5 is a cross-sectional view showing the wafer of FIGURES 1-4 still mounted upon the glass plate with gold plate deposited over the surface opposite the glass plate after removal from the plating tank shown in FIG- URE 4;

FIGURE 6 is a cross-sectional view of the device of FIGURE 5 showing how it would appear when completed;

FIGURE 7 is a cross-sectional view showing a device similar to that of FIGURE 3 which is to be plated in accordance with an alternative embodiment of the present invention;

FIGURE 8 is a view similar to FIGURE 4 showing the device of FIGURE 7 suspended within a plating tank;

FIGURE 9 is a perspective view showing a silicon wafer deposited within a plating tank in order to carry out another alternative embodiment of the method of the present invention; and

FIGURE 10 is a cross-sectional view of the wafer of FIGURE 9 showing how it will appear upon completion.

Referring now'to the drawing and more particularly to FIGURE 1, there is shown a diode in an intermediate stage of production and which includes a monocrystalline silicon body 11 which has an interior zone 12 of N-type conductivity which may, for example, be the result of the presence of arsenic introduced into the silicon during the crystal growing process. About the surface of the body 11 there is a thin P-type zone 14 formed, for example, by the diffusion of boron therein in accordance with the method described and claimed in United States Patent No. 2,827,403, issued March 18, 1958 Upon the surface 15 the large area front gold contact is to be formed in accordance with the method of the present invention. Such a connection may advantageously be formed over the entire surface 15 by the following method.

In this example it will be assumed that the wafer is cylindrical in shape as shown and has a diameter in the range from inch to one inch and an overall thickness of approximately fourteen thousandths of an inch. Further, in the example herein described the resistivity of the single crystal silicon material is in the range of from 5-15 ohm centimeters. While this invention will be described in connection with the production of a front ohmic contact over the entire surface 15 of the wafer, it will be understood that the invention is equally applicable to the production of either a forward or back contact upon any type of semiconductor device, including diodes, transistors, photo-transistors, photo-cells and the like.

In order to produce the ohmic contact consisting of gold over the surface 15, the wafer 11 is first heated to a temperature in the range from 50 C. to 60 C. by, for example, placing it, with surface 15 down, upon a hot plate (not shown) whose temperature may be regulated. With the wafer disposed upon the hot plate with surface 15 resting thereupon, a solution of argentous fluoride is prepared by beginning with a solution of argentous fluoride which consists of 50% by weight of argentous fluoride in water. This starting solution is then diluted to one part to 121 parts of distilled water by volume. While the concentration of the argentous fluoride solution, in accordance with the method of the present invention, has been found to be particularly satisfactory when the ratio is 1:121 as hereinabove stated, the concentration may vary from as little as 1:60 to 1:250. This diluted solution is preferably maintained at room temperature. With the silicon wafer 11 resting with surface 15 on the hot plate and maintained at a temperature between the range from 50 C. to 60 C., the opposite surface 16 of the wafer is coated with the diluted argentous fiuoridesolution by brushing it thereover. The wafer is maintained at this elevated temperature until the argentous fluoride solution evaporates to thus permit a coating of elemental silver 17 to be deposited over surface 16 of the wafer 11 as may best be seen in FIGURE 3. Typically, the thickness of the silver coating 17 will be of the order of approximately 10- inches. A glass plate 20 as shown in FIGURE 5 has placed thereupon a gold foil 21 of a thickness of approximately one thousandth of an inch and a width and a length approximately one-half and two inches, respectively. The gold foil 21 is fastened to the glass plate 20 by glueing, for example. A glue or cement which has been found to be satisfactory for this purpose is cement No. 910 manufactured by the Eastman Kodak Co. of New York. A wire or electrode 22 is attached to one edge of the gold foil 21 by welding, soldering, or the like. The wafer 11 with the silver coating 17 facing downward as shown in FIGURE 5 is placed in contact with the gold foil 21. Next, a plating mask which may be of a material such as a vinyl plastic is placed about the periphery of wafer 11 and forms a fillet shaped ring 27 thereabout while holding the wafer 11 in place upon the gold foil 21.

The extending wire 22 is now connected to a source (not shown) of negative potential of the order of magnitude of 40 volts D.C. While for this particular example, 40 volts has been found to be a satisfactory bias voltage, any voltage in excess of one volt will be suflicient. What is desired is that the voltage be greater than the silver solution potential in contact with the cyanide ions which will be present in the plating bath hereinafter to be discussed. This is necessary in order to preclude the possibility of having a dissolution of the silver during the gold plating operation subsequently to be described. With the negative potential applied to the wire 22 the entire wafer 11, while mounted upon the glass plate 20, is suspended within a plating tank 40 as may best be seen in FIGURE 4. The tank 40 is then filled with a solution of potassium gold cyanide which is preferably produced by the following technique. Approximately ten grams of potassium gold cyanide (including 67%% gold by weight), together with approximately fifteen grams of potassium cyanide are dissolved in approximately 1000 ml. of distilled water. This entire solution is preferably brought up to a temperature of approximately C. within the tank 40 and there maintained during the gold electroplating operation which is to follow. An anode electrode 42 separated from the glass plate 20 is provided within the tank 40 and is connected to a source of positive potential (not shown) in order to induce a plating current of between 80-100 ma. The current density may vary between 3 amperes per square foot to 1-1 amperes per square foot. A coating of gold 44 will thus be electro-plated upon the surface 15 of the wafer 11 as the remaining surfaces of the silicon water are masked by the glass plate 20 and the cement 27. The gold plating is permitted to continue for approximately five to ten minutes after which the silicon wafer still mounted upon the glass plate is removed from the plating bath. There will thus be deposited by this electro-plating technique a uniform layer of gold 44 as shown in FIGURE 6, over the surface 15 of wafer 11 which has a thickness of approximately 0.008 inch.

The silicon wafer 11, while still mounted upon the glass plate 20, is next placed within an oven (not shown) with the gold side up. A second glass plate or ceramic plate (not shown) is then placed over the gold layer 44 and a stainless steel weight of approximately three ounces is placed over the second ceramic or glass plate. A forming gas consisting of approximately 85% nitrogen and 15% hydrogen is passed 'through'the oven which is-heated to a temperature of between 380 C. and 550' C. and preferably 450 C. for approximately 30 minutes in order to sinter the .gold layer 44 into the silicon surface 15. The wafer 11 is now removed from the oven and is ready for dicing into individual diodes in a manner well known to the art. Prior to the dicing, the silver coating 17 and its underlying P-type region off the surface 16 will preferably be lapped off.

In FIGURES 7 and 8 there is shown an alternative apparatus for carrying out the method of the present invention as described in connection with FIGURES 1-6. In accordance with this alternative method a silver coating 17 is deposited upon the surface 16 of the wafer 11 from a solution of argentous fluoride as hereinabove described. After the silver coating has been permitted to dry, a contacting clip 56 which has a reverse action due to the spring 57, is employed to grip the wafer 11 for suspension within the tank 58 as shown in FIGURE 8. A wire or electrode 59 is connected to the contacting clip 56 for connection to a source of negative potential. Subsequently, the gold plating operation is carried out in the same manner as described in connection with FIG- URE 4 as discussed hereinabove with anode electrode 62 being disposed within the tank resulting in gold coating 65 on surface 66 of the wafer 11.

In order to point out some of the advantages inherent in the method of the present invention it would be well to consider what is believed to be the physical phenomena which occurs in the plating technique herein described. Ordinarily, it is difficult to electro-plate a conductive metal upon a body of semiconductive material due to the fact that semiconductive material such as silicon; for example, has a relatively low electrical conductivity. If one attempts to plate a conducting metal such as gold, for example, to the surface of a semiconductive material, it is ordinarily necessary to apply one or more electrodes to the body of semiconductive material to be plated. Because of the relatively high resistance of the semiconductive material the plating will tend to be thicker along the surface of the body of semiconductive material opposite the electrode or electrodes, as the resistance path elsewhere will be great with respect to these local areas. Thus, the plating will be uneven in thickness when produced by such a technique.

The essence of the present invention involves the predeposition of a layer of a broad area of high conductivity material, namely silver, which permits a spreading of the electro-plating current over the entire surface of the body to be plated. It should further be pointed out that the silver which is deposited upon the body of semiconductive material by wetting it with argentous fluoride may easily be removed from thesurface of the body merely, for example, by simple mechanical abrasion. Thus, predetermined patterns of uniform gold plating may be achieved by establishing a predetermined pattern of a coating of silver. Thus, by first depositing silver over an entire surface of the body and then removing it in accordance with a given pattern, or by first masking the surface to be coated with silver in accordance with a given-pattern, various patterns or shapes of gold may be produced on a surface of a body of silicon.

A technique for producing a ring shaped gold, contact, for example, will now be described. A silver coating in the shape of a ring 70 may be deposited over the surface 71 of the silicon Wafer 72shown in FIGURE 10. The ring 70 may be produced bythe technique hereinabove described which would involve the formation of a coating of silver over the entire surface 71 and its subsequent removal by abrasion over all of the surface except the ring by use of a template which has a ring shape coincident with the ring 70. Alternatively, the surface '71 m y be masked prior to the wetting of the surface withthe argentous fluoride solution so that a ring shaped coating .71 will be deposited initially. Thereafter, the wafer '72 may be suspended as shown in FIGURE "9 within tank .80 which includes asolution of potassium gold cyanide which is in accordance with that described with reference to FIGURES 4 and 8. A contacting electrode 76 which may be attached to the silver ring by a technique similar to that described in FIGURE 4, will have applied thereto a :negative bias through wire 76, while a-second spaced electrode 78 will be positively biasedcausing the electroplating of a gold ring 81 upon the surface 82 .of FIGURE 10 which is in effect, a shadow of .the silver ring 70. By a similar technique various shapes and patterns of gold or other contacting metals may be provided upon the surface of a semiconductor body in accordance with the method of the present invention.

Other possible uses of the method of the present invention include the following: A silver coating may be deposited over the surface of a semiconductive body by immersion of the body in a solution of argentous fluoride. Thereafter gold may be electro-plated over the silver coating and if it should be desired, over the entire surface of the body.

The method of the present invention may also be used as an intermediate step in making an etch resistant mask in the production of a semiconductor device which does not necessarily form an ohmic bond with the semiconductor material.

There has thus been described a new and improved method for producing large area contacts to the surface of a semiconductor body. In particular, this method has been described in connection with the production of an etch resistant ohmic contact to be used as a front contact for a semiconductor diode. It has been found that by applying the method of the present invention for producing such a front contact, for a diffused junction silicon diode of the character described, that the forward current capacity of a typical device has been improved from 30 ma. at +1 volt to 800 ma. at +1 volt without any other change in the device characteristics.

While argentous fluoride has been designated herein as the particular silver salt employed in accordance with the method invention, other silver salts may also be used. Such silver salts must be capable of dissolving silicon by forming a stable complex therewith, which complex will not permit the precipitation of the silver salt therefrom. Such salts include, in addition to argentous fluoride, silver salts in the presence of a high concentration of fluoride ions and hydrofluoric acid. Examples of such are silver oxide plus hydrofluoric acid; silver sulphate plus hydrofluoric acid; silver nitrate plus hydrofluoric acid and silver acetate plus hydrofluoric acid. All of these react to form silver fluoride with the same molar ratio as the argentous fluoride hereinabove designated.

It will be clear to one skilled in the art that the thinner the semiconductive body and the higher the resistivity thereof, the more accurately will be the electro deposition of the gold contact. Further, by varying the time of immersion, the temperature of the cyanide bath and the amount of current in a manner which may be determined by one skilled in the art, the thickness of the gold coating may be accurately controlled.

As has been indicated hereinabove metals other than gold may be plated either upon the silver coating produced in accordance with the method of the present invention or on the other surface or surfaces of the semiconductive body. Other metals which may be so plated include silver and palladium, for example.

What is claimed as new is:

l. The method of depositing a conducting metal upon 7 a' predetermined portion of a body of semiconductive material including the steps off wetting the semiconductive body with an aqueous solution of argentous fluoride to thereby deposit upon said bodya coating of silver, said solution consisting of one part of argentous fluoride to from about 60 to 250 parts of distilled water, the temperature of said solution being maintained in the range from 50"v C. to 60 C.; removing said coating of silver from all but a first predetermined portion of the surface of said body; contacting an electrode to said coating of silver; and thereafter electro-plating a conducting metal upon a second surface of said body opposite said first surface upon which said silver coating is deposited in accordance with the pattern of said coating of silver. 7

2. The method of depositing a conducting metal upon a predetermined portion of a body of semiconductive material including the steps of: wetting the semiconductive body with an aqueous solution of argentous fluoride to thereby deposit upon said body a coating of silver, said solution consisting of one part of argentous fluoride to about 123 parts of distilled water, the temperature of said solution being maintained in the range from 50 C. to

' 60 C.; removing said coating of silver from all but a References Cited in the file of this patent UNITED STATES PATENTS Waltz Nov. 26, 1957 Bradley Aug. 5, 1958 

1. THE METHOD OF DEPOSITING A CONDUCTING METAL UPON A PREDETERMINED PORTION OF A BODY OF SEMICONDUCTIVE MATERIAL INCLUDING THE STEPS OF: WETTING THE SEMICONDUCTIVE BODY WITH AN AQUEOUS SOLUTION OF ARGENTOUS FLUORIDE TO THEREBY DEPOSIT UPON SAID BODY A COATING OF SILVER, SAID SOLUTION CONSISTING OF ONE PART OF ARGENTOUS FLUORIDE TO FROM ABOUT 60 TO 250 PARTS OF DISTILLED WATER, THE TEMPERATURE OF SAID SOLUTION BEING MAINTAINED IN THE RANGE FROM 50*C. TO 60*C. REMOVING SAID COATING OF SILVER FROM ALL BUT A FIRST PREDETERMINED PORTION OF THE SURFACE OF SAID BODY; CONTACTING AN ELECTRODE TO SAID COATING OF SILVER; AND THEREAFTER ELECTRO-PLATING A CONDUCTING METAL UPON A SECOND SURFACE OF SAID BODY OPPOSITE SAID FIRST SURFACE UPON WHICH SAID SILVER COATING IS DEPOSITED IN ACCORDANCE WITH THE PATTERN OF SAID COATING OF SILVER. 